Humans normally have 46 chromosomes (two copies - one from each parent - of each of the 24 chromosomes: [1:22] + [XX or XY]).

Aneuploidy is an abnormal number of chromosomes - Down's syndromes is an example of this, where individuals have an additional copy of chromosome 21.

I have read a little about gender-related aneuploidy, where individuals can have additional sex-specific chromosomes (for example XXY).

I was wondering what (roughly) the prevalence of the different combinations are, what the specific phenotypes are, and what (if any) medical complications can arise from these combinations, or indeed, what advantages could be conferred by the additional chromosome!

47,XXX

47,XXY

47,XYY

My question also stretches to whether the different combinations display a hermaphroditic phenotype?

1 Answer
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Note that anyone with a Y chromosome is considered a genetic male. Also, aneuploidy is usually a result of nondisjunction (but can have other causes as well) and is usually not inherited.

47,XXX is called Triple X syndrome and occurs in approximately 1 in 1,000 female births. These individuals usually appear normal, but may have tall stature, a small head, and delayed development. According to this case report, the tall stature is possibly due to SHOX gene triplication, in some cases. The short stature homeobox gene (SHOX) gene is a gene on the X and Y chromosome and is associated with short stature if a copy of the gene is missing or mutated. The case report states, "The enhancing effect of SHOX gene triplication on growth was ascribed to the concomitant absence of gonadal steroids in cases of gonadal dysgenesis, allowing a prolonged period of growth."

47,XXY is called Klinefelter's syndrome. Here is an image/diagram of symptoms. These people may have gynecomastia (enlarged breasts), less muscle, less body hair, and hypogonadism (decreased testicular function, also resulting in smaller testicles and penis) due to lower testosterone production. These symptoms become more obvious after puberty, because individuals with Klinefelter's have less "masculinization" (for lack of a better word). Cognitive problems are usually present. It occurs in roughly 1 in 500 to 1 in 1,000 male births.

47,XYY is XYY syndrome and has an incidence of 1 in 1,000 male births. They appear normal but may be slightly taller than average. This study looked at the cognitive and motor phenotypes of boys with Klinefelter's and XYY syndromes. It found that both groups have generalized cognitive impairment, but XYY has more profound language impairment and Klinefelter's has more significant motor impairment. Their hypothesis is that "the more severe language-based cognitive phenotype in XYY versus KS is likely to be genetically determined on the basis of abnormal dosage of specific Y chromosome genes, but not abnormal levels of testosterone because XYY is not associated with testicular failure."

There's also 45, X0 (Turner syndrome), in which girls have a webbed neck, broad chest, low-set ears, short stature, gonadal dysfunction, and often other conditions such as a bicuspid aortic valve, coarctation of the aorta, and vision problems. Image of symptoms. It occurs in 1 in 2,000 live births. Individuals usually have 1 X chromosome, but may also have 2 X chromosomes, with 1 of them being incomplete (source). The single copy of the SHOX gene (mentioned above) has been linked to the short stature in Turner syndrome.

Interestingly, people with 47,XYY and 47,XXX people often have normal fertility, but not in the other two cases. My uneducated guess is that the extra "normal" chromosome (for the person's sex) doesn't disrupt gonadal function.

Interesting that 47,XXX and 47,XYY both display a 'tall' phenotype, but otherwise appear normal. Any idea why this might be? You mention that 47,XXY individuals are defined as male due to the presence of the Y, but do they display feminine traits other than enlarged breasts? And what cognitive problems might occur? The more detail the better - had hoped for more than just wiki links :) Thanks
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LukeAug 29 '12 at 13:20

I added a little more detail. I'm not sure how 47, XXX individuals are taller, but I can see that the extra Y in 47, XYY individuals make them more masculine. Just a guess though. I'll see if I can find more info about how these disorders cause their cognitive and developmental effects.
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jelloAug 29 '12 at 14:14

@Luke Tried to answer some of your questions in the edits. I'm not sure about the cognitive problems though :( I hope someone else can add to it.
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jelloAug 29 '12 at 18:41